1. Field of the Invention
The present invention concerns a process for the elimination of sulfate ions from impure phosphoric acid. More particularly, the present invention concerns the elimination of sulfate ions during the purification of impure phosphoric acid by means of a liquid-liquid extraction process. According to the invention, a purified and highly desulfated phosphoric acid is obtained from a raw wet process phosphoric acid, said purified acid being usable in the human food industry.
2. Description of the Prior Art
It is known to prepare phosphoric acid according to a wet process comprising the attack of a strong acid such as sulfuric acid on phosphate ore. After separation by filtration of the insoluble residue and of the gypsum formed, there is obtained an impure solution of phosphoric acid containing numerous anionic and cationic impurities, particularly sulfate ions originating from the excess sulfuric acid which is introduced during the attack on the ore so as to obtain maximum yields in the solubilization of the calcium phosphate. To obtain a relatively purified aqueous solution of phosphoric acid, a subsequent liquid-liquid extraction process is usually applied to the raw acid utilizing an organic solvent which is substantially insoluble in water, such as alkyl phosphates or aliphatic alcohols having 4 to 9 carbon atoms. In the course of this extraction, in view of the partition coefficient of the acids between the aqueous phase and the solvent phase, a considerable portion of the sulfate ions follow the phosphate ions into the organic phase. After washing the organic phase containing the acids with water or a dilute phosphoric acid, the organic phase is re-extracted with water to yield an aqueous solution of purified phosphoric acid, which still has a sizable sulfuric acid content. Thus, in French Pat. No. 1.531.487 in the name of Jean Goret and Louis Winand (corresponding to U.S. Pat. No. 3,607,029, incorporated by reference herein in its entirety and relied upon), there is obtained at the completion of the process, after extraction with tributyl phosphate in the presence of excess sulfuric acid (excess sulfuric acid being utilized in order to improve the yield in the extraction of the phosphoric acid), a phosphoric acid solution having an H.sub.2 SO.sub.4 content with respect to P.sub.2 O.sub.5 of 0.5% or 5000 ppm. Operating under critical conditions, this process provides at best a content by weight of SO.sub.4 --in relation to P.sub.2 O.sub.5 of 500 to 1000 ppm. However, the food industry increasingly requires a highly desulfated phosphoric acid containing less than 100 ppm SO.sub.4 by weight with respect to P.sub.2 O.sub.5. Such is notably the case for certain nonalcoholic beverages, since an SO.sub.4 content in excess of 100 ppm is prohibited by current legislation.